One example of the conventional motor-operated valve of this kind is shown in FIG. 3. The motor-operated valve 10′ shown therein is provided with a valve chamber 21, a valve seat 22 (a valve aperture 22a), and a valve body 20 having a flange-like member 23. The valve body 20 is constructed such that the flow rate of fluid such as refrigerant can be adjusted by a valve plug 24a (a valve stem 24) which is enabled to move close to or away from the valve seat 22. A top-closed cylindrical can 40 having an opened bottom is hermetically connected, through a lower end 40b thereof, with the flange-like member 23 (through a step portion 23a provided thereon) of valve body 20 by means of butt welding.
The valve chamber 21 of valve body 20 is communicated, through one side portion thereof, with a refrigerant inlet pipe 61 and also communicated, through a bottom portion thereof, with a refrigerant outlet pipe 62.
A rotor 30 is coaxially positioned in the can 40 with a predetermined gap being formed between the outer circumferential wall of the rotor 30 and the inner circumferential wall of the can 40. A stator 50 comprising a yoke 51, a bobbin 52, stator coils 53 and a resin mold cover 56 is disposed around the outer circumferential wall of the can 40, thereby constituting, together with the rotor 30, a stepping motor.
A driving mechanism for enabling the valve plug 24a to move close to or away from the valve seat 22 by making use of the rotation of the rotor 30 is provided between the rotor 30 and the valve stem 24. Specifically, this driving mechanism is constituted by a feed screw mechanism 15 which comprises a cylindrical guide bush 26 (having a stationary threaded portion 25 formed on the outer circumference thereof) which is fixed, through a lower end portion 26a thereof, to the valve body 20 and has an axial hollow portion in which the valve stem 24 is permitted to slidably move, and a cylindrical valve stem holder 32 (having a movable threaded portion 31 formed on the inner circumferential surface thereof and adapted to be engaged with the stationary threaded portion 25) which has an opened lower end and is coaxially disposed around the outer circumferential surfaces of the valve stem 24 and the guide bush 26.
More specifically, an upper portion of the valve stem 24 is slidably inserted into an upper hollow portion of the valve stem holder 32 and the top portion of the valve stem 24 is fastened by means of nut 33. As the valve stem holder 32 is rotationally moved up and down by means of the feed screw mechanism 15, the valve stem 24 is caused to correspondingly move up and down without being rotated together with the rotation of the valve stem holder 32.
According to the motor-operated valve 10′ which is constructed as described above, when the stator coils 53 are excited through the application of electric current thereto in one direction under a condition where the valve plug 24a is kept away or lifted from the valve seat 22 (valve aperture 22a is opened), the rotor 30 and the valve stem holder 32 are caused to rotate in one direction relative to the guide bush 26 which is fixedly secured to the valve body 20, thereby causing the valve stem holder 32 to move downward by means of the feed screw mechanism 15, whereby the valve plug 24a is enabled to press-contact with the valve seat 22, thus closing the valve aperture 22a. 
Even at this moment where the valve plug 24a is seated on the valve seat 22, a movable stopper 37 provided at a lower end portion of the valve stem holder 32 is not yet contacted with a stationary stopper 27 fixed to the valve body 20, so that the rotor 30 and the valve stem holder 32 are still permitted to rotationally move downward with the valve aperture 22a being kept closed by the valve plug 24a. On this occasion, since the valve stem holder 32 is moved downward relative to the valve stem 24, a buffer coil spring 34 interposed between the valve stem holder 32 and the valve stem 24 is compressed. As a result, the valve plug 24a of valve stem 24 is strongly pressed onto the valve seat 22, thus closing the valve. Thereafter, when the rotor 30 is further rotated to cause the valve stem holder 32 to move downward, the movable stopper 37 is caused to press-contact with the stationary stopper 27, so that even if the application of electric current to the stator coils 53 is continued, the rotation and downward movement of the valve stem holder 32 can be forcedly suspended.
Under this suspended condition of the valve stem holder 32, when the stator coils 53 are excited through the application of electric current thereto in the other direction, the rotor 30 and the valve stem holder 32 are caused to rotate in a direction, which is opposite to the aforementioned one direction, relative to the guide bush 26. As a result, the valve stem holder 32 is caused to move upward by means of the feed screw mechanism 15, thereby enabling the valve plug 24a to move away from the valve seat 22 to open the valve aperture 22a. As a result, the refrigerant that has been introduced into the valve chamber 21 from an inlet pipe 61 is permitted to flow via the valve aperture 22a into an outlet pipe 62. In this case, the flow rate of refrigerant can be adjusted depending on the magnitude of lift of the valve plug 24a (for details, see JP Laid-open Patent Publication (Kokai) No. 2001-50415).
In the case of the conventional motor-operated valve 10′ which is constructed as described above, the flow rate of refrigerant is determined depending on the effective area of aperture of the valve aperture 22a, i.e. on the magnitude of lift of the valve plug 24a from the valve seat 22. The magnitude of lift of the valve plug 24a is determined depending on the pitch of the feed screw mechanism 15 and on the rotational speed of rotor 30 (the number of steps of the stepping motor).
In the case of the motor-operated valve of this kind, the maximum magnitude of lift (a fully opened state) of the valve, in other words, the maximum number of steps of stepping motor is generally determined in advance for enabling a desired maximum flow rate to be obtained. Namely, the motor-operated valve is designed such that the control of electric current to the stators can be performed so as to enable the rotor to rotate five revolutions at maximum for example. In this case, if the pitch of the feed screw mechanism 15 is set to 0.6 mm for example, the magnitude of lift of the valve plug 24a can be changed at intervals of 0.6 mm, i.e. 0.6 mm, 1.2 mm, 1.8 mm, 2.4 mm, 3.0 mm, as the rotor 30 and the valve stem holder 32 are caused to rotate one revolution, two revolutions, three revolutions, four revolutions and five revolutions, respectively.
Meanwhile, in the case of the motor-operated valve to be employed in a refrigerator (to be employed as an expansion valve) in particular, since the flow rate of refrigerant to be controlled is very small, it is difficult to finely control the flow rate of refrigerant. Further, when carbon dioxide gas is to be employed as a refrigerant, it is required to increase the pressure of refrigerant to a high pressure (about 10 times as high as the conventional pressure). However, there is a problem that when a high-pressure refrigerant is employed with the valve plug being lifted at intervals of 0.6 mm as described above, the fluctuation of flow rate of refrigerant would become too large.
It may be conceivable to minimize the size or angle (effective area of aperture) of the valve aperture as one of countermeasures for solving the aforementioned problems. However, there is a limitation in decreasing the effective area of aperture of valve and moreover, even if such a countermeasure is employed singly, it would not lead to full solution to the aforementioned problems.
It may be also conceivable, as another countermeasure, to minimize the pitch of the feed screw mechanism 15 (for example, to decrease the pitch of 0.6 mm to 0.2 mm). However, when the pitch of feed screw mechanism 15 is decreased, the magnitude of lift per the revolution of the rotor would become too small, thereby raising a problem that the stationary stopper 27 for regulating the rotation may not be suitably contacted with or moved away from the movable stopper 37.